What will it take to convince a panel of experts that a diet pill is safe, not just effective? Vivus is the latest company to fail to come up with an answer. An FDA advisory committee today voted 10 to 6 to reject its Qnexa pill, which was effective in clinical trials, but worried panelists about usage by pregnant women and psychiatric side effects. These concerns were raised by agency medical reviewers (look here), given that diet drugs are expected to be widely used on a long-term basis.

The outcome contradicts what some Wall Street analysts were cautiously predicting - a vote in favor of recommending approval, albeit with labeling and marketing restrictions, which would have made Qnexa the first new prescription diet pill in more than a decade. However, this has been a three-way race. Two other small companies are hoping to win FDA approval for their diet drugs - Arena Pharmaceuticals, which just struck a deal with Eisai (see here) - and Orexigen Therapeutics. FDA panels will vote on those later this year.

But today’s vote may dampen enthusiasm for the whole group. Arena and Orexigen shares, for instance, both fell substantially in late-afternoon trading. “We expected the Qnexa vote to be an uphill climb with a close negative outcome following Avandia. I think the climb (for the diet drugs) is equally steep,” Ramsey Baghdadi, a pharma analyst at Concept Capital, tells us. “The panelists were not naive that this (drug) will get into broad populations without restrictions. The firms need to go to school on the REMS,” or Risk Evaluation & Mitigation Strategies.

The diet pill market, by the way, is littered with failure. The 1997 withdrawal of one of the two pills in the fen-phen weight-loss cocktail and, Redux, a chemical cousin, focused national concerns on the debate over safety, sufficient weight loss and risks. Since then, Abbott added cardiovascular warnings to its Meridia pill; Roche added warnings about liver injury to Xenical (which is now sold over-the-counter as Alli by GlaxoSmithKline) and Sanofi-Aventis’ Acomplia never made it to the US market due to psychiatric side effects.

pic thx to alan cleaver on flickr